Liquid crystal devices comprise two generally parallel substrate surfaces that are spaced apart by a cell gap filled with a layer of liquid crystal material to form a liquid crystal cell. The substrates can be conditioned on their inner surfaces to define the alignment of the liquid crystal directors contacting these surfaces. Use of liquid crystal devices is prevalent in display systems. In one category of liquid crystal display, electrodes for applying a longitudinal electric field are located on the inner surface of each substrate, the liquid crystal material has a positive dielectric anisotropy, and the substrate inner surfaces are conditioned to cause the liquid crystal directors to align parallel to the surfaces or at a small angle relative to them. An example of such a display would be the reflective, self-compensating twisted nematic (SCTN) mode display described by K. H. Yang, Eurodisplay, 449-451 (1996). The SCTN display has a twist angle from 60° to 65°, and the polarization direction of incident linearly polarized light bisects the twist angle of the SCTN cell. The name self-compensating is chosen because, in the electrically switched ON state, which is the optically dark state, the phase shift introduced between the linear polarized light components by the retardation of the upper boundary layer is modified by the retardation of the lower boundary layer such that the polarization components are again in phase. An SCTN mode display is reported by Yang to have a contrast ratio exceeding 270. Although a contrast ratio of 270 was considered in 1996 to be a high value, it would now be considered inadequate for front or rear projection TV applications, in which contrast ratios of over 2000 are commonplace.
In another category of liquid crystal displays, electrodes for applying a longitudinal electric field are located on the inner surface of each substrate, the liquid crystal material has a negative dielectric anisotropy, and the substrate inner surfaces are conditioned to cause the liquid crystal directors to align vertically, or nearly vertically, to the plane of the substrate surfaces and thereby form a surface tilt angle or pretilt angle up to 90°. These types of displays are referred to as Vertically Aligned (VA) mode, homeotropic, or quasi-homeotropic displays and promise higher contrast ratios than those available in the SCTN mode. This category of display can operate in either the transmissive mode or the reflective mode. An example of such a transmissive mode display would be the screens used in many of the currently available flat panel computer monitors and TVs. Reflective mode displays include certain Liquid Crystal on Silicon (LCoS) imaging devices that are used in near-eye and projection applications.
When the liquid crystal directors contacting the substrate surfaces are aligned perfectly vertical to the substrate surfaces, the surface noncontacting directors throughout the layer, including the layer midplane, are also aligned perfectly vertical. For this special case, there is no birefringence imparted to light propagating along the direction normal to the surfaces of the substrates. Light leakage in this state can be very small across a pair of crossed polarizers, leading to an extremely high contrast ratio, because it is limited only by effectiveness of the light polarizer system used. However, this perfectly vertical director configuration is not practical for displays because, when an electric field is applied to tilt the directors to switch the display to the ON or optically bright state, there is no defined direction for the directors to tilt. This tilt ambiguity leads to unpredictable domain lines and dark regions throughout the layer.
This tilt ambiguity can be overcome by conditioning the substrate surfaces to decrease the pretilt angle from 90° to a smaller value, thereby breaking the symmetry and producing the so-called quasi-homeotropic director configuration. Such a pretilt angle can be generated, for example, by coating the surface with a special polymer, such as the SE-1211 alignment polymer available from Nissan Chemical Industries, Ltd., and unidirectionally rubbing it with a velvet cloth. Alternatively, the substrate surface can be conditioned by vacuum deposition of a material such as SiO2 from one or more oblique angles. Applying an electric field to such a quasi-homeotropic structure results in a predictable and well-defined director field throughout the entire liquid crystal layer, leading to a uniformly bright display free from any domains or dark regions. But the introduction of a pretilt angle of less than 90° reduces the display contrast ratio because light propagating normally to the substrate surface planes encounters in-plane retardation, which introduces light leakage in the electrically switched OFF or optically dark state.
Although a pretilt angle very close to 90° is sufficient to break the vertical symmetry, an 85° or even smaller pretilt angle is generally needed in practical displays. This is so because in a practical display device, whether it is a direct view TFT LCD device or a small form factor LCoS imaging device designed for HDTV, the display is comprised of many small pixels and the electric fringe fields generated between the pixels can cause the surface noncontacting liquid crystal directors to tip in the wrong direction and introduce objectionable disclination domains. These domains introduce not only dark, objectionable patterns in a bright pixel, but also very long electro-optic responses that are highly undesirable and cause such phenomena as “tailing.” Smaller pretilt at substrate surfaces will tend to suppress the appearance of the disclination lines that result from fringe fields. However, even pretilt angles in the 85°-88° range also cause significant decrease in contrast ratio. Such decrease in contrast ratio is likely to be even greater for LCoS imaging devices because the larger fringe fields generated by the smaller pixels may require pretilt angles as small as 75° to suppress the objectionable domains.
To make the situation worse, the OFF state of the quasi-homeotropic display is generally operated not at 0 volts but rather at a subthreshold drive voltage, V0, to secure an optimal ON state drive voltage, V1, that lies within the dynamic range limitations V1-V0 of the drive circuitry. A non-zero subthreshold voltage introduces a further decrease in contrast ratio because, lacking a true threshold, the surface noncontacting directors are tilted even more than they would be at zero volts, which further increases the in-plane retardation.